"Astronomers on the New Horizons team, who sent a spacecraft to Pluto in 2006, are very unhappy. The thing their craft is going to is no longer a planet. It’s sort of fun to tease them about that."Image: Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute

Astronomer Mike Brown played a role in booting Pluto out of the planetary pantheon.Image: Nasa, JPL-Caltech

Mike brown set out to expand the galaxy, not shrink it. He spent much of his childhood surveying the night sky, hunting for alien worlds in the outer reaches of the solar system. Then he grew up to become a planetary astronomer at Caltech and really did find something. He called it Eris, but being only slightly bigger than Pluto and three times as far from the sun, it just didn’t seem worthy of planet status. And this caused a major problem. If Eris didn’t qualify as a planet, then neither did Pluto. The result: Pluto officially lost its planet title in 2006. In his new book, How I Killed Pluto and Why It Had It Coming, Brown describes his sordid role in the biggest astronomical controversy since Sinbad’s appearance on Star Search. We asked him how it feels to be a destroyer of worlds.

How many planets are there? Just eight.

Because you killed Pluto. Yes.

Why does Pluto not deserve to be a planet? Pluto as an individual is absolutely not as important as the other eight planets. If it were to disappear suddenly tomorrow, it would not change anything dramatic about the solar system. We wouldn’t even feel it. You can’t say that about the eight real planets.

Might there be another ninth planet out there? I predict eight will stay the number forever. Maybe there’s something larger than Mercury in the very distant part of the solar system that’s in a crazy elliptical orbit. But if we found something like that, we would have to rethink the definition of a planet all over again.

This all started when you found what you took to be a new planet. It was what I had been looking for the entire time—something bigger than Pluto! The 10th planet! But it never felt right. Finding Eris wasn’t like finding Neptune or Uranus, because those planets are thousands of times more massive. And they’re also thousands of times more massive than Pluto. So we had to give Eris the boot, and that meant Pluto had to go, too. I didn’t think anyone had the guts to do it, but it was the right decision scientifically.

Who is the most upset by the delisting of Pluto? Astronomers on the New Horizons team, who sent a spacecraft to Pluto in 2006, are very unhappy. The thing their craft is going to is no longer a planet. It’s sort of fun to tease them about that.

What does your 5-year-old daughter think of having a planet-killer for a father? She has learned from general discussions that I killed Pluto and that killing is bad. Therefore, I’ve done something bad, and so she’s kind of mad at me.

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